Reserve Estimation of Barite Deposits Using Geological and Geophysical Investigations in Cross River State South Eastern Nigeria

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1 Reserve Estimation of Barite Deposits Using Geological and Geophysical Investigations in Cross River State South Eastern Nigeria DOMINIC.A.OBI, BARTH.N.EKWUEME AND. GAGRIEL.B.AKPEKE DEPARTMENT OF GEOLOGY UNIVERSITY OF CALABAR ABSTRACT Barite deposits occur mostly as concealed mineralization dominantly in shales and sandstones of Cretaceous age in Cross River State. Vertical Electrical Sounding (VES) and Horizontal Resistivity Profiling (HRP) have been carried out in areas with less mining activities. Also, field geological mapping in which the lengths and the widths of barite veins were measured has been undertaken.the geophysical data collected within 22 communities was done in 52 locations and the results indicate that the barite occurs in veins located at m depth and could be up to 40m in open mine pits. The lateral extent of the veins range from 4m to 65m from the from the horizontal resistivity profiling results, and could be traced from the field mapping to about 950m. The general direction of the barite mineralization is NE-SW. The result of the reserve estimate indicate that the Agoi village of Oban Massif has 1,981,177.0 metric tons of reserves, these are greater than the reserve found at mamfe embayment villages of Ekukunela and Nkarasi with reserve deposits of about 865, 684 metric tons. These reserves are also greater than those found in parts of the lower Benue trough villages of Osina and Okpoma (Ogoja) with reserve deposits of about 774, 345 metric tons. An estimated reserve of over nine (9) million metric tons of barite have been calculated and documented for Cross River Stat for the first time. This has unarguably put Cross River State as having the largest reserve of barite in Nigeria. KEY WORDS: Barite, VES, HRP, Reserve, geophysical. INTRODUCTIOIN Barite can occur in three forms, which include stratiform or bedded, residual and in veins. Barite veins occur in many parts of the world and they are the most productive. For example, such veins yielded 900 tons of barite in North Africa in 1994; 2,400 tons of barite in Middle and Far East in the same year and 355 tons barite in South Africa. Kaiser et al. (1987) described barite deposits in veins mostly metamorphosed in rocks of the Piedmont belt, Virginia U.S.A. Barite veins occur widely in the United States of America and other parts of the world. Residual deposits are formed when disseminated barite or barite veins in pre-existing rocks accumulate on a basement as the previous host rock is eroded and weathered. The important deposits of Washington County occur in residual clay derived from Cambrian dolomite in which primary deposits occur (Bateman, 1950) Barite mineralization in Nigeria was first reported by the Geological survey of Nigeria. No interest was shown then towards its exploration. Detailed exploration work on barite deposit located at Azara District part of Nasarawa State in northcentral Nigeria was carried out by the Nigerian Mining corporation between 1975 and 1980 and a reserve of about 730, 000 tonnes of barite was established over the area (Maiha, 1996). As at 2004 the Federal Ministry of Solid Minerals Development (FMSMD 2004) has documented only two States with barite mineralization in Nigeria. Cross River State, this study area, was not listed. Only Plateau State (Azara) and Taraba State (Iba and Dumgel) both in north central Nigeria were listed. This study reports abundant occurrence of barite deposits in Cross River State of Nigeria. Geophysical Survey was employed to unravel most of the barite deposits which occur as concealed vein mineralization. The geophysical technique has made it possible to estimate the reserve in the area which is up to 9 million metric tonnes greater than barite reserves for the well-known Azara occurrence which has a reserve of 730,000 tonnes. GEOLOGICAL SETTING Two giant spurs make up the Precambrian basement of southeastern Nigeria, namely the Oban massif and the Obudu Plateau (Fig 1). These spurs are the western prolongation of Cameroon Mountains into the Cross river plains of Southeastern Nigeria. The basements are overlain by Cretaceous sediments of the Calabar Flank in the south and west but separated by a Cretaceous sediment filled graben or Mamfe rift (Embayment) in the north (Fig 2). Orajaka (1964), Umeji (1988), Ekwueme (1990), Ekwueme and Onyeagocha (1982), Ekwueme (2003), Ukwang (1998), Ukaegbu (2003), Ephraim (2005), and Obioha and Ekwueme (2011) have studied in detail the Precambrian basement rocks in the area. These are composed of phyllites, schists, gneisses, granulites and migmatites intruded by rocks of granitic, mafic and ultramafic composition. They range in age from NeoArchaean to Pan-African (Ekwueme and Kroener 1997, 1998). 18

2 A dolerite in Obudu yielded 40 Ar/ 39 Ar plateau age of 140 ± 0.7Ma (Ekwueme 1994a). The basement has undergone polyphase deformation and polymetamorphism and several generations of folding, faulting, shearing and fracturing have been reported (Ekwueme 1987, 1994b). The dominant trend of the structural features comprising of planar and linear types is N-S to NE-SW ( ). Minor trends in the NW-SE and E-W also occur and have been interpreted as relicts of pre-pan African deformation episodes (Grant 1972; Onyeagocha and Ekwueme, 1982; Ekwueme 1987, 1994b). Bassey (1998) had suggested that the Oban massif and Obudu plateau could have been continuous prior to the formation of the Mamfe Basin which lies between them (Petters et al. 1982). Fig 1: Geologic Sketch Map of South-Eastern Nigeria (After Ekwueme et al. 1995). 19

3 Fig 2: Geological Map of Calabar Flank, Oban Massif and Mamfe Embayment (After Ekwueme et al., 1995) The Mamfe Embayment (Fig 2) situated between the Oban massif and Obudu Plateau is predominantly a fluviatile clastic sequence that exhibits point bar fining-upward cycles and over bank mudrocks (Ekwueme, 2003). This formation has been described as the Asu River Group of Albian age. Associated with the 20

4 sedimentary rocks of Mamfe rift are basaltic rocks which exhibit excellent columnar joints in the middle of the lava flow. The type locality of the Mamfe Formation is on the bank of Cross River at Mamfe in adjoining Cameroon Republic where 800m of massive arkosic sandstones with marl, sandy limestone and shale intercalations are exposed (Reyment 1965). The sequence in the Nigerian part of the Mamfe Basin comprises conglomeritic immature mudstones (Petters, 1982, Ekwueme 1994) have shown evidence of faulting at the edge of the Mamfe Basin. The Calabar Flank (Fig. 2) comprises of lithologies such as sandstone, limestone, marl and shale. The oldest formation in the Flank is a sandstone shale sequence which is folded and lies unconformably on the Precambrian basement at Awi. This is the Awi formation of Aptian-Albian age described by (Petter, 1982). Overlying these rocks is the Mfamosing Limestone Formation. The Eze-Aku shale lies on the Mfamosing limestone. It has been described as Ekenkpon shale by Petters (2003). Associated with these shales are bioturbatred marls which are overlain by Nkporo shale of Campanian-Maastritchtian age. The youngest formation in the Calabar Flank is the unconsolidated sand belonging to Benin Formation of Tertiary age. It is deltaic. FIELD OCCURRENCE AND DISTRIBUTION OF BARITE MINERALIZATION Structural features are most important in the localization of the barite ore and have greatly influenced barite mineralization in the study area. The detailed features discovered to determine the immediate localization of barite in the area are mainly unconformities and major faults. These are features that come under daily observation of the mining geologists. They become well exposed in mining operation and are observed slowly but destroyed rapidly. The major faults of the Benue Trough and the unconformities along the boundary between the basement and the sediments are favourable sites for the accumulation of the barite deposits in the study area. This is why some deposits are found to occur in lenses and impervious covers. Due to tectonic events that took place in the study area, layers of rocks have been folded into curves that are lower in the middle than at the end known as geosyncline. Obviously in a geosyncline, accumulation of sediments was followed by uplifting, faulting and folding. This is evident from the location of barite vein within the generalized stratigraphic chart at Agoi Ibami (Fig 3) in which the mineralization is concentrated within the sediments at the boundary between the basement and the sediments. These sediments are the carbonaceous shales, limestone, siltstone, and sandstone with the Cross River and Asu River Groups of the Albian sequence in the lower Benue Trough (Fig 4). The Cenomanian was a period of uplifting and non-deposition in the most part of the Benue Trough and at this time the Albian sediments in the Trough axis were slightly deformed, fractured, brecciated and intruded by mafic dykes (Nwachukwu 1972, Benkhelil 1987). In the Turonian, the Asu River Group was covered by sediments of the Eze-Aku Formation consisting of carbonaceous shales, limestones and siltstone which grade into sandstone (Akande et al. 1992). Hence, it is hereby suggested that the mineralization episodes were in pulses occurring in basement, in cross-cutting intrusive and in sediments. For instance, barite deposits found in Agoi Ibami occur in calcareous sandstones and carbonaceous shales with high permeability that helped to localize the mineral in these areas. The calcareous sandstone therefore allowed the mineralizing fluids to form barite deposits in this area. Overlying the basement complex in some parts of the study area are Cretaceous sediments of the Benue Trough within which the barite mineralization occurs (Fig 5). The sketch of typical barites bearing dolerite dyke that occupies the boundary between sandstone and basement is shown in Fig 6. 21

5 Fig 3: Location of barite vein in a generalized stratigraphic chart at Agoi Ibami ( N, E) (Scale; 1cm=0.1m for stratigraphic sequence 1cm=50m for barite vein length) 22

6 Fig 4: Generalized Cretaceous Stratigraphy of the Lower Benue Trough (Simplified from Akande et al, 1989) 23

7 Fig 5: Generalized model of Barite Mineralization in Cross River State (Scale: 1cm = 50m for barite vein length ) 24

8 Fig 6: Sketch of barite bearing dolerite dyke occupying the boundary between sandstone and basement at Okurike ( N, E) (Scalae: 1cm = 50m for barite vein length) GEOPHYSICAL SURVEY Geophysical survey was carried out in areas with minimal interference by mining activities, whilst some physical parameters were used to evaluate barite deposits in other areas where mining activities could not permit geophysical survey. Electrical Resistivity of Gabu, Osina and Alifokpa areas was carried out using an Abem Terrameter (300 SAS model) and employing the Vertical Electrical Sounding (VES) and Horizontal Resistivity Profiling (HRP) techniques. The apparent resistivity of each subsurface rock layer can be calculated from the following relationship: 25

9 Where k is the geometric factor and R is the resistance of the rock layer (dobrin et,al 1988) from Ohm s Law, Where V is the potential difference measured across the two potential electrodes and is the current through the two current electrodes. If half current electrode spacing which is usually the same as the depth of investigation is given as L, and half the potential electrode spacing is given as a, then for the Schlumberger array, the geometric factor, K can be calculated from the relationship, (Telford et, al 1990). and The Schlumberger configuration was employed in carrying out the vertical electric sounding (VES) and the horizontal resistivity profiling (HRP). The VES and HRP traverses were made in a more or less NE-SW direction in both areas. The VES technique was based on Schlumberger electrode array using a maximum half current electrode spacing of 215m. The current electrode spacing (AB/2m) was varied from m at logarithmically equal intervals giving a depth of investigation of 100m or more. In each case, the expansion of the current electrodes was in a direction parallel to the average strike (NE-SW), of the outcrops. The apparent resistivities (pa) versus (AB/2) were plotted digitally and the resultant sounding curves were smoothened and then interpreted by curve matching techniques.(griffith et, al 1981). 26

10 Fig. 7 Modelled Resistivity Sounding Curve for Osina (VES) FFFFffffl ff 27

11 Fig.8 Modelled Resistivity Sounding Curve for Gabu (VES) 28

12 Fig.9 Modelled Resistivity Sounding Curve for Alifokpa (VES) 29

13 Table 1: Result of modeled VES Layers showing depth and Resistivities values OSINA 01 METERS 13 Resistivities GABU 01 METERS 13 Resistivities ALIFOKPA 01 METERS 13 Resistivities

14 Depth (m) 0 GABU SECTION OSINA SECTION ALIFOKPA SECTION Resistivity (σ ) Resistivity (σ) Resistivity (σ) 36.3 Depth (m) 0 CM 2188 GLS Depth (m) SCM SSM 201 SSM 244 SSM BZ GM BZ GM BZ GM Fig 10 Geoeletric sections showing depths of zones of suspected Barite Mineralization at Osina, Gabu, and Alifokpa (BZ barite zone, GLS = Gravel / Laterite soils, CM = clayed material, GM = Granite material SCM = saturated clayed material) SSM = saturated sand material Number represent apparent resistivities in Ohm m. 31

15 Resistivity ( Zb: suspected zone of barite mineralization Zb Zb Station Interval (m) Graph Of Wenner Array Profiling Along Osina, Gabu And Alifokpa Scale x: 1cm = 20 units y: 1cm = 500 units Legend Osina Gabu Alifokpa Fig 11: Graph of Wenne Array Profilling along Osina, Gabu and Alifokpa 32

16 The VES measurements were carried out to determine the variation of rock resistivity with depth and then the top and bottom surface of the concealed barite mineralization. The HRP was carried out using Wenner electrode array with a constant station interval of 10m. Twenty two (22) stations spaced at an interval of 10m were occupied along the HRP lines at Gabu, Osina and Alifokpa. The choice of the electrical resistivity method was based on the anomalous high resistivity (Egeh, et al, 2004) usually associated with most mineralized ores like barite, which generally exhibit high densities and extremely low porosities. The resistivity measuresments made in the field were then subjected to interactive computer modeling, as well as manual interpretation to delineate zones with anomalous resistivity in relation to the geology of the area. GEOPHYSICAL INTERPRETATION The resistivity measurement made in the field at 22 location using the vertical electric sounding VES and horizontal resistivity profiting HRP. However only three locations of Osina, Gabu and Akofakpa are displayed for illustration on how the data was processed. The field measurements are analysed using the Zoody Software in processing the VES data and grapher 2.0 in processing the HRP data. The values for AB/2 and MN/2 were manually keyed into the programme and subjected to iterative computer modeling (Figs 7, 8 and fig 9 respectively). The result of iterative curve matching of the observed curve and the calculated curve is displayed in table 1.0. Which is used to produce the geoelectric sections fig.8. The results of the geomodels indicates that the three modeled sections has four subsurface layers each as shown in fig.10 There is a correlation in the resistivities values within the subsurface layers. Low resistivity values indicates areas with clays moderality high values represent sandy clay s with possible saturation of ground water while areas of moderately high resistivity values represent possible barite refrailtored granites with barites. Intrusives while very high resistivity values represent fresh basements of granitic origin. The Gabu section has a top layer clay materials with resistivity of 36.3Ωm to depth of 4:6m, below this unit is the sandy day materials with resistivity values of 380Ωm at depth α 14.7m, this unit is underlain by possible fresh barite of high resistivity value of 3992Ωm at depth of 31.8m, below this zone is the fresh granite materials with resistivity values of 15444Ωm. The Osina model section has a top layer of consolidated gravels and lateritic soils with a high resistivity value of 2188Ωm, this is followed by a moderately high saturated sandy clayed layer with a resistivity value of 210Ωm, this layer is followed by a possible weathered barite zone with a resistivity of 595.8Ωm which is underlain by a granitic basement with a resistivity value of Ωm. Also the Alifokpa model section indicates a top saturated clay unit with a resistivity value of 19.1Ωm, this is followed by a compacted sandy clay unit with a resistivity value of Ωm basement with a resistivity value of 3899Ωm. The barites in the area possibly intruded the granitic materials which extends to 11.5m in Gabu, 1500m in Osina and 152m in Alifokpa (table 2.0). 33

17 TABLE 2: BARITE EVALUATION FROM RESISTIVITY STUDY AT OSINA, GABU AND ALIFOKPA COMMUNITIES COMMUNIT LAYER DEPT THICKNE THICKNE LATERA AREA Y/ NUMBE RESISTIVITI RANG SS SS OF L EXTEN VES SITE R ES Ωm E (M) (m) PROSPEC EXTENT T (m) T AREA (m) (m) Gabu Osina Alifokpa TOTAL 2757M 2 34

18 TABLE 3: ESTIMATION BARITE RESERVES OF THE MAIN VEIN IN STUDY AREA LOCATIONS WIDT H (W) 1 TRACEABL E LENGTH (M) 2 APPAREN T DEPTH (M) 3 OVER DURDE N (M) REAL DEPT H (M) 5 VOLUME(M 3 ) 1X2X5 6 BARITE RESERV E IN (TONS) 6 X SG ALIFOKPA (AL) OSINA(OS) V , , , ,042 8,848 14, , , , , , V3 V4 GABU V5 (GA) , , , , , , V3 OMOJI (OM) OKPOMA (OKP) EKUKUNELA (EK) ATAKPA (AT) V3 NKARASI (NK)

19 EDONDON (ED) OKOKORI (KO) V 2.20 DOME 900 OF RADIUS D OKUMURETE T (ET) V IYAMITETE (IY) V AGOI IBAMI NORTH (AN) V3 V4 AGOI F. R.(AE) V3 AGOI (AE) EKPO ITU AGOI (IT) UGBEM (UG) AKPET 1 (AK) V3 OKURIKE (OK) V3 IBOGO (IB)

20 V3 NSAKWAN (NS) NDE (ND) TOTA L 9,793, MILLIO N TONS EVALUATION OF BARITE MINERALIZATION Just few weeks after the completion of geological mapping of the study area, local and illegal miners were found to have spread almost all over the entire area. This really hindered the smooth running of the geophysical survey. However, geophysical survey was carried out in the areas with minimal mining activities such as Osina and Gabu. In areas where intensive mining activities had commenced, the tonnage mined and that still left were both estimated for total computation of barite deposit in the area.. The detailed computation of the barite reserves was done in considering the following: BT = Width Traceable length * Real depth * Specific gravity Where BT = Barite in tones Width = measured width of barite veins in the field Traceable length = lateral extent of barites estimated from HRP or field outcrops. Read depth = depth estimates to the barite deposit within the host rock, this is acquired from results of VES apparent depth overburden depths or from locally mined outcrops. Specific gravity = Specific gravity of different barite samples measurement in the laboratory in the different sites, from previous studies they range form The above computation Indicated that Barite reserves within the Oban Massif is greater that occurring within the Mamfe embayment and the parts of the lower Benue trough. The reserves within Oban Massif include Agoi villages has a total reserve of 1,981, metric tons, 1, 786, 876 metric tons there reserves are greater than those found in parts of the lower Benue trough which include Okpoma (Ojoja) 496, 626 metric tons, Gabu 250, metric tons and Osina 277, metric tons. Also, the Mamfe Ekukunela village 544, metric tons Nkarasi 320, 689 metric tons (Fig 13). The total amounts of barite deposit in Cross River State is estimated to be about 9,66,306 tonnes (nine million sixty six thousand Three hundred and six metric tons). CONCLUSION Mining activities for concealed barite deposits has been ongoing in different parts of Cross River State on small scale local mining in hand dug trenches/pits this research is a pioneer work in trying to estimate the total reserve for the state. The research employed geoelectrical surveys (VES and HRP), geological field mapping, and some laboratory measured parameters in evaluating the total barite deposits in Cross River State in excess of 9 million metric tones the highest in Nigeria. Its our hope that the high specific granites ( ) which is higher than the oil companies standard of 4.2 and the large volume in excess of 9 million tons will attract investor in the oil exploration industries to exploit the large reserves of barite deposits in Cross River State, Nigeria. REFERENCES Akande, S. O., Andreas, A. and Berndu, E Environment of ore formation and Anchizonal metamorphism in Pb-Zn fluorite- barite deposits of the Benue Trough, Nigeria. Geol. Mijnb. 71: Bateman, M. A Economic Mineral Deposit. 2 nd Ed. Wiley & sons Inc. London. Benkhelil, J Creteaous deformation, magmatism metamorphism in the lower Benue Trough, Nigeria, Geol. J., 22:

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